pooh if it's at the Con and 'real people' involved we'll still need a Discworld theme so we do still need a venue for it to be based in and to provide plausible good/bad roles? Also with potentially higher-volume participation there's a lot more scope for those factions for the Townies and you could also have 2 (or more) sets of Mafia/Assassins/Werewolves depending on take-up - running the game could get very convoluted indeed!

"Some men see things as they are and ask why. Others dream things that never were and ask why not.” George Bernard Shaw

I'm not talking about a real Discworld Con.
The game would be set at an imaginary Discworld Con.
Then non-players can just come up and speak to you at anytime eg;
"Oooh look! There's that Pact-Breaker Sjoerd. My God, he's tall"

"Disliking Carrot would be like kicking a puppy.""You kicked a puppy," Lobsang said accusingly.